The actions of many extracellular signals are mediated by the interaction of G-protein coupled receptors (GPCRs) and guanine nucleotide-binding regulatory proteins (G proteins). G protein-mediated signaling systems have been identified in many divergent organisms, such as mammals and yeast. GPCRs respond to, among other extracellular signals, neurotransmitters, hormones, odorants and light. GPCRs are similar and possess a number of highly conserved amino acids; the GPCRs are thought to represent a large ‘superfamily’ of proteins. Individual GPCR types activate a particular signal transduction pathway; at least ten different signal transduction pathways are known to be activated via GPCRs. For example, the beta 2-adrenergic receptor (βAR) is a prototype mammalian GPCR. In response to agonist binding, βAR receptors activate a G protein (Gs) which in turn stimulates adenylate cyclase and cyclic adenosine monophosphate production in the cell.
It has been postulated that members of the GPCR superfamily desensitize via a common mechanism involving G protein-coupled receptor kinase (GRK) phosphorylation followed by arrestin binding. Gurevich et al., J. Biol. Chem. 270:720 (1995); Ferguson et al., Can. J. Physiol. Pharmacol. 74:1095 (1996). However, the localization and the source of the pool of arrestin molecules targeted to receptors in response to agonist activation was unknown. Moreover, except for a limited number of receptors, a common role for β-arrestin in GPCR desensitization had not been established. The role of β-arrestins in GPCR signal transduction was postulated primarily due to the biochemical observations.
Many available therapeutic drugs in use today target GPCRs, as they mediate vital physiological responses, including vasodilation, heart rate, bronchodilation, endocrine secretion, and gut peristalsis. See, e.g., Lefkowitz et al., Ann. Rev. Biochem. 52:159 (1983). GPCRs include the adrenergic receptors (alpha and beta); ligands to beta ARs are used in the treatment of anaphylaxis, shock, hypertension, hypotension, asthma and other conditions. Additionally, spontaneous activation of GPCRs occurs, where a GPCR cellular response is generated in the absence of a ligand. Increased spontaneous activity can be decreased by antagonists of the GPCR (a process known as inverse agonism); such methods are therapeutically important where diseases cause an increase in spontaneous GPCR activity.
Efforts such as the Human Genome Project are identifying new GPCRs (‘orphan’ receptors) whose physiological roles and ligands are unknown. It is estimated that several thousand GPCRs exist in the human genome. With only about 10% of the human genome sequenced, 250 GPCRs have been identified; fewer than 150 have been associated with ligands.